LEDs have become increasingly popular as a lighting choice and, for many applications, have begun to replace conventional lamps having filament. For example, LEDs are now widely used in traffic signal lights and for the back lighting of liquid crystal display (LCD) panels.
In many applications, it is it desirable to vary the lighting output (i.e., brightness) of an LED. Generally, it is difficult to use voltage control for controlling LED brightness. Instead, the brightness of an LED is proportional to its current. Therefore, LED current should be controlled to control LED brightness (e.g., for dimming an LED). With the increased popularity of LEDs in numerous applications which require varying degrees of brightness, there is an increasing need for a suitable power converter for controlling LED current.
In some applications, the power for driving LEDs is in the form of an alternating current (AC) input. In this case, the AC line current needs to be synchronized to the line voltage, thus minimizing line current distortion so that the transferred energy from the power source is maximized. If there is a phase delay between the incoming voltage and current, the transferred energy is circulated from source to load. This reduces the power transferred from the source to the load in relation to the cosine of its phase difference. If the voltage and line current are made to be in phase, the phase difference is zero and its cosine becomes unity. This technique is known as power factor correction (PFC). Sometimes, the line current may be distorted and harmonics are involved in the line current by power conversion processing.
According to some previous designs, power converters for LEDs require at least two power stages in order to provide both control of LED current as well as power factor correction (PFC). Each power stage performs some form of power conversion. Typically, the first stage is referred to as a pre-regulator and provides PFC control. The second stage is a DC-to-DC converter and provides LED current control. Because any given power stage is not 100% efficient, there is a loss of power across each stage for such power converters. This results in lower overall efficiency for the power converter. For example, assuming that the each of two power stages in a previous design power converter is 90% efficient, then the overall system efficiency will be 81% efficient (0.90×0.90=0.81).